1. Field of the Invention
This invention relates generally to radio frequency (RF) receivers and, more particularly, to compensate undesirable DC offset present in mixers.
2. Related Art
To optimally process radio frequency (RF) signals, most RF receivers convert received RF signals to lower frequencies termed baseband frequencies. The filtering and amplification performed in processing the RF signal at baseband frequency requires less expensive electrical components than those required for accurate processing at RF. Typically, RF receivers employ mixers to convert the received RF signal to a lower frequency while conserving the modulation information contained in the received signal. Frequency shifting occurs by mixing, or taking the difference between, the received RF signal and a reference frequency from a local oscillator (LO). The difference between the RF signal and the LO frequency is the lower frequency or baseband frequency.
The process of converting the RF signal to a lower frequency is called downconverting. The RF receiver functions to downconvert a received RF signal to baseband. Direct conversion receivers directly downconvert the received RF signal to baseband by mixing the received RF signal with a LO frequency equal to the received RF frequency.
An example of a conventional mixer is the commonly known xe2x80x9cGilbert cell.xe2x80x9d The mixer includes a plurality of transistors and a RF input section. Input to the Gilbert cell is a LO signal, such as a voltage, where the oscillation of the LO signal causes a current to commutate between pairs of transistors. This commutation action produces a mixing of the RF signal with the local oscillator signal to produce an output baseband signal that is a downconverted received RF signal. The baseband signal contains the information of interest. In some communication standards (including many wireless standards), information is present at direct current (DC).
When a differential signal is used, there is always a DC offset present. As mentioned above, in some standards, the desired information is present at DC. Thus, the inherent DC offset present in the circuit must be extracted or removed from the DC present in the desired information. There are several mechanisms in which the DC offset may introduce undesirable interference in a direct conversion receiver. The first is the LO leakage (coupling) back to the RF input due to the proximity of the LO circuitry to the RF circuitry. Since LO strength is typically kept constant, the resulting DC offset from LO leakage does not change with time. Such an offset is known as a static DC offset.
Second, interferer signals, also known as blockers, may leak onto the LO input. Examples of interferer signals include, but are not limited to, signals from other nearby communication devices using portions of the frequency spectrum that are relatively close to the frequency spectrum employed by the RF signal that is downconverted by the direct conversion receiver. Another common interferor in wireless standards is from the transmitting unit in the same communicating device. The signal leaking onto the LO input that is associated with the intereferor signal is known as an interferor leakage signal. Therefore, the interferer leakage signal on the LO input mixes down with the interferer signal present at the RF input of the mixer to yield a DC offset at the output of the mixer that varies with interferer signal strength. Because the resulting DC offset varies with the strength of the interferer signal, this DC offset is a dynamic DC offset.
Third, even-order nonlinearities in the mixer may introduce significant distortion in direct conversion receivers. The direct conversion receiver employs a plurality of transistors fabricated on a single chip. If the physical parameters including area and doping concentrations of transistors are not symmetric, then the differences, however small, results in the transistors performing differently from the ideal transistor. In practical applications of conventional integrated circuit (IC) transistor on-chip fabricating processes, all transistors are at least slightly different from each other in that no two transistors can be identically fabricated down to the molecular level. Mismatches in the transistor pairs of a Gilbert cell mixer yields two types of DC offsets at the mixer output: static and dynamic. Static DC offsets are dealt with easily using techniques and apparatus well known in the art. However, dynamic DC offsets caused by the even-order nonlinearities are not adequately compensated for in conventional systems. The fourth interference mechanism occurs from mismatches at the direct conversion receiver mixer load. This interference mechanism, yields static DC offsets. Compensation for this interference mechanism is well known.
Within the communication industry, significant efforts continue to attempt to minimize the undesirable interference and/or signal distortions caused by the above interference mechanisms resulting from the DC offset. Therefore, there is an on-going need to continue to further minimize the impact of these mechanisms, and particularly, the dynamic DC offsets associated with interferor signals (the second and third mechanisms above), so that direct conversion receivers for wireless standards may be fabricated using IC technologies.
The invention is a DC offset compensator that compensates for dynamic direct current (DC) offsets resulting from the interferer signal accompanying an incoming RF communications signal interacting with the associated interferer leakage signal appearing on the local oscillator (LO) input to the mixer, as well as dynamic DC offsets associated with the even-order nonlinearities caused by differences in transistors fabricated on the mixer chip.
In one embodiment, the DC offset compensator is employed in a mobile communication device. An incoming radio frequency (RF) communications signal is mixed with a local oscillator signal (LO) in a direct conversion mixer. The incoming communication signal, at times, has both a communication signal of interest and other communication signals, known as interferor signals (also known as blocker signals). The interferer signal induces undesirable DC offset at the output of the mixer via two mechanisms: self-mixing between the interferor signal and an associated interferer leakage signal, and interaction with even-order nonlinearities present in the mixer.
The DC offset compensator detector detects the presence of the interferer signals and provides a compensating correction to the output of the mixer such that some or all of the undesirable DC offset caused by the interferer leakage signals are removed (compensated out) of the mixer output signal. In one embodiment, the DC offset compensator detects the presence of interferer signals directly from the incoming RF communication signal. Other embodiments of the DC offset compensator detect changes in the common mode current of the mixer. Other embodiments of the DC offset compensator are employed in other devices that have receivers, such as, but not limited to, televisions, radios, stereo receivers, satellite receivers, or the like. Furthermore, alternative embodiments of the DC offset compensator are coupled to other types of circuitry in a system having undesirable DC offsets.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.